> you could reject the message and show the user an error but only if there's a discrepancy of >X minutes
No you can't, not in a federated and decentralised system like this.
The sender can wait for a read receipt from a given receiver user, if the receiver is willing to make those public. But if the message left client A and didn't arrive at client B, there's no objective fact of the matter about whether the message "was sent" or not.
